Knowing when to harvest your grapes

Tom van der Linden

Time: 6:26 | This video shows how to assess the ripeness of your grapes through sight, touch, smell, and taste; and by measuring sugar content, pH, and acidity level.

Video transcript

[Tom van der Linden, University of Minnesota Extension educator]: It's a beautiful September day in Minnesota. We're at the Horticultural Research Center at the University of Minnesota and it's time to harvest the grapes. But how do you know when it's time to pick? I'll show you how to tell when the grapes are ripe and I'll show you some measurements to be sure, because it's important. After all, better grapes mean better wine.

The most important day of the year is the day you pick. It sets in motion the annual harvest and it also determines the kind of wine you'll make. As a grape grower and a wine maker you should keep a notebook. You'll be glad to have records in the months and years to come.

Let's start with sight, touch, smell, and taste. You want your grapes to be rich in color, not green. A ripe grape will crush easily, but not be shriveled. A ripe grape is plump, and thickly juicy. It's a balance between sweet and tart. Each variety develops special flavors that we call varietal flavor. A fully ripe grape develops its varietal flavor more fully. Does the skin have varietal flavor? Is it herbaceous, or is it vegetal, like a green pepper? Is the aftertaste pleasant or is it bitter? Chemical or vinegar tastes or smells are flaws so take good notes if you have that problem. One more time, taste the grape and imagine what the wine will be. Then we'll move on to some lab work.

It's good to use your senses but it's also important to measure. You need to measure sugar content, pH, and acidity level. Grapes are mostly water and sugar which will ferment to make wine. Brix is a term that the brewing industry uses to measure the sugar content of grapes. Brix level helps estimate the alcohol level of your wine. Like temperature, Brix is measured in degrees. Brix is measured with a refractometer, which you can buy at a winemaking supply store or online. Drop some juice on the test plate, close the cover firmly and look through the viewfinder. You'll see a line where your juice registers on an internal scale. In this case, the juice registers 24 degrees Brix.

A somewhat less convenient, but cheaper method is to buy a simple glass hydrometer which has a built-in scale. Simply pour your juice into the cylinder, float your hydrometer and read the Brix level right off the built-in scale. The more sugar in your wine, the higher your hydrometer will float. As your grapes mature, they store more sugar so the Brix level rises. Different wine styles require different Brix levels. In general, for white wine, 22 Brix is good. We'll keep an eye on our grapes, testing them periodically, and when we reach our Brix goal, then it's time to pick.

We now know about sugar and how to measure it. Next let's quickly shift to pH and the pH meter. You may remember pH from high school science class. It's a measure of free hydrogen ions. As our grapes ripen and the sugar rises, the pH will rise too. You can buy an inexpensive, portable pH meter. Be sure you buy pH reference solutions so you can calibrate your meter.

Grape juice is full of natural acids, which lend important qualities to wine. Every time we measure Brix we should also measure acid levels. In a way they're opposites; as the Brix goes up, the acid levels go down. You can buy a simple acid test kit. It takes a little practice and a little care and you'll want to make good records, but don't worry, you can do it!

So enough with measuring and tasting, let's go pick!

Are you ready to pick? Let's start by picking a good sample. Yes, you can pick one grape, put it in your refractometer and take a sugar sample, but it won't be very representative of all your grapes. Instead, pick individual grapes from many clusters. Sample from both sides of the vine, high and low, in sunny areas and shaded areas, and pick from different parts of each cluster. An ideal sample might be 50 grapes. But if you only have a few vines, it'll be okay to take a smaller sample. Make a note about how they felt, how they smelled, and how they tasted. Then, squeeze your juice into a cup. Use the juice to make your measurements, and then write down your results. Enjoy picking your grapes, then measure a sample for sugar, acidity and pH and record it. Then, sort through your grapes. Pick out any green, moldy or shriveled berries and you're ready to start making wine.